The present application is based on and claims priority under 35 U.S.C xc2xa7119 with respect to Japanese Patent Application No.2001-326779 on Oct. 24, 2001 (13th Year of Heisei), the entire content of which is incorporated herein by reference.
1. Field of the Invention
The present invention is generally directed to a multi-stage vacuum pump and in particular to a multi-stage vacuum pump whose inner structure is improved such that in-vacuum-pump substances which are easy to coagulate are made free from solidification by utilizing heat of compression resulting from generates upon gas compression.
2. Prior Art
In general a conventional multi-stage vacuum pump of the type is constructed to have a plurality of in-series pumping chambers each of which accommodates a pair of intermeshing rotors which are all of a xe2x80x9cRootsxe2x80x9d-type profile. The pair of xe2x80x9cRootsxe2x80x9d-type rotors which are provided in each pumping chamber is rotated therein to make a space evacuated which is connected to an inlet port or suck port of the pumping chamber by compressing a gas sucked from the space to be evacuated. While the rotors are being in rotation, a heat of compression is generated due to the gas compression, which is cooled by water or air in order to prevent a temperature increase of a housing of the multi-stage vacuum pump.
While the multi-stage vacuum pump is in operation, the resulting compression load generates a heat of compression, which increases the temperature of the housing of the multi-stage vacuum pump. As well known, the heat of compression becomes much larger at an exhaust or outlet port than the inlet or suck port, in resulting very large temperature differential therebetween.
In the conventional multi-stage vacuum pump, a specific gas to be exhausted such as ammonium chloride is brought into condensation or solidification at ordinary temperature or its near region as viewed from saturation vapor pressure curve. Thus, when such a specific gas is sucked, the resulting gas is cooled down, in the pumping chamber of earlier stage which is relative low in temperature, below a temperature of solidification, which causes the gas to solidify or condense, resulting in a deposit at a portion such as an interface between the rotor and the housing in the pumping chamber, whereby drawbacks may occur such as pump overload when the rotors in rotation and/or stopping the rotation of each of the rotors.
For example, in Japanese Patent Publication No. 3051515 provides a multi-stage vacuum pump whose structure is shown in FIG. 9. In this structure, a common cooler 47 is provided to cool down each of different stage pumping chambers from which gases are exhausted with heat generation. In addition, in this structure, four pumping chambers 42, 43, 44, and 45 are defined in a housing 40 such that its lower wall portion 46 closes exhaust ports of the respective pump chambers 42, 43, 44, and 45. The lower wall portion 46 is connected with a cooler 47 through which a cooling water passes, which establishes an indirect cooling of the housing 40 via the lower wall portion 46.
In xe2x80x9cRootsxe2x80x9d-type vacuum pumps, in general noise generation is at issue which results from particular that the Roots profile rotors in the chamber adjacent the pump outlet expelling discrete trapped volumes of evacuated gas to atmosphere from between the Roots rotors. To prevent the generation of such noise, in the conventiol xe2x80x9cRootsxe2x80x9d-type vacuum pumps, a silencer is provided in a spaced manner from the pump housing.
However, in the above-described conventional xe2x80x9cRootsxe2x80x9d-type vacuum pump, the compression heat generation is caused by the compression work at each of pumping chambers such that the compression work becomes larger at higher stage pumping chamber. Thus, the temperature of the gas sucked into the inlet port is made larger as being transferred to higher stage pumping chamber in the order the first-stage, second-stage, third-stage, and fourth-stage pumping chambers, resulting in that the temperature of the gas becomes maximum near or at the outlet port of the fourth-stage pumping chamber. This causes a thermal gap or temperature differential between the inlet port and outlet port. Consequently, if the gas which is to be evacuated contains therein a condensable gas such as ammonium chloride is brought into condensation or solidification at ordinary temperature or its near region as viewed from its own saturation vapor pressure curve. Thus, when such a gas is sucked, the resulting gas is cooled down, in the pumping chamber of earlier stage which is relative low in temperature, below a temperature of solidification, which causes the gas to solidify or condense, resulting in a deposit at a portion such as an interface between the rotor and the housing in the pumping chamber, whereby drawbacks may occur such as pump overload when the rotors in rotation and/or stopping the rotation of each of the rotors.
In addition, adding the silencer to the conventional xe2x80x9cRootsxe2x80x9d-type vacuum pump results in an increase of the number of parts, an increase of production cost, and an increase of mass or outer scale.
Thus, a need exists to provide a xe2x80x9cRootsxe2x80x9d-type multi-stage vacuum pump which is free from the above-described drawbacks, which is capable of, by simple structure, exhausting sucked gas without solidifying the same, and which is made, at a lower cost, free from compression noise upon gas exhaustion.
Accordingly, in order to meet the above need to overcome the aforementioned drawbacks or problems, a first aspect of the present invention provides a multi-stage vacuum pump which comprises:
a housing in which a plurality of pumping chambers are formed, the pumping chambers being arranged in series and being in fluid communication with one another, one of the pumping chambers which is at one end of the series acting as an initial stage pumping chamber, another of the pumping chamber which is at the other end of the series acting as a final stage pumping chamber,
the housing being provided with an inlet port for sucking a gas from a space to be evacuated into the initial stage pumping chamber, the housing being provided with an outlet port for exhausting the gas from the final stage pumping chamber;
a Roots-type pump section occupying each of the pumping chambers; and
first means for decreasing a temperature differential between the initial stage pumping chamber and the final stage pumping chamber.
A second aspect of the present invention is to provide multi-stage vacuum pump whose gist is to modified the structure of the first aspect, wherein the inlet port and the outlet port of the housing are placed near the initial stage pumping chamber, the first means is in the form of a passage connecting between the inlet port and the outlet port.
A third aspect of the present invention is to provide a multi-stage vacuum pump whose gist is to modify the structure of the second aspect, wherein the passage extends along a lengthwise a of the housing.
A fourth aspect of the present invention is to provide a multi-stage vacuum pump whose gist is to modify the structure of the second aspect, wherein the passage is modified to act concurrently as a built-in silencer.
A fifth aspect of the present invention is to provide a multi-stage vacuum pump whose gist is to modify the structure of the first aspect to comprise further second means for cooling a heat of compression generated at each of the pumping chambers.
A sixth aspect of the present invention is to provide a multi-stage vacuum pump whose gist is to modify the structure of the fifth aspect, wherein the second means is in the form of one more cooling fluid flowing passages which are so formed in the housing as to be near the first means.
A seventh aspect of the present invention is to provide a multi-stage vacuum pump whose gist is to modify the structure of the sixth aspect, wherein the cooling fluid flowing passage is a tube which is in thermal contact with the housing.
An eighth aspect of the present invention is to provide a multi-stage vacuum pump whose gist is to modify the structure of the fifth aspect, wherein the second means is in the form of fins formed integrally with the housing.
A ninth aspect of the present invention is to provide a multistage vacuum pump whose gist is to modify the structure of the first aspect to comprise further a check-valve provided in the outlet port.
A tenth aspect of the present invention is to provide a multi-stage vacuum pump whose gist is to modify the structure of the fourth aspect, wherein making the passage to have different inner diameter forms the built-in silencer.
An eleventh aspect of the present invention is to provide a multi-stage vacuum pump whose gist is to modify the structure of the fourth aspect, wherein making the passage curved forms the built-in silencer.
A twelfth aspect of the present invention is to provide a multi-stage vacuum pump whose gist is to modify the structure of the fourth aspect, wherein providing a sound-absorbing material in the passage forms the built-in silencer.